The effect of pretreatment with α-methyl-p-tyrosine (α-MT) on the basal levels of plasma growth hormone (GH) and the responses to chlorpromazine (CPZ) were investigated in urethane-anesthetized rats with either complete hypothalamic deafferentation (CD.), hypothalamic ablation (H.A.) or sham operation (Sham). Basal GH levels were high in C.D. rats, intermediate in H.A. rats, and low in Sham rats without any pretreatment. Pretreatment with α-MT caused a significant increase in basal GH levels in both C.D. and Sham rats, but not in H.A. rats. GH release following the intravenous injection of CPZ, which was observed in C.D. and Sham rats without α-MT pretreatment, was blunted by treatment with α-MT. In H.A. rats CPZ failed to stimulate the secretion of GH regardless of α-MT pretreatment. Neither the injection of L-DOPA nor L-DOPA affected basal GH levels in non-α-MT pretreated C.D. rats. However, plasma GH levels significantly decreased following the injection of L-DOPA but not L-DOPA in C.D. rats pretreated with α-MT. These findings suggest that the injection of CPZ causes an enhancement of GH release by inhibiting the catecholaminergic (dopaminergic) mechanism, which is active within the basal medial hypothalamus (BMH) and plays an inhibitory role in GH secretion. They also suggest that the extrahypothalamic inhibitory neural pathway, which is connected to the BMH and is interrupted by hypothalamic deafferentation, is not catecholaminergic.